This invention relates to the manufacture of fine particles of phosphor for electroluminescent (EL) lamps and, in particular, to a process for treating previously coated phosphor particles to improve electrical stability without impairing brightness.
An EL panel is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is a transparent metal layer, such as indium tin oxide (ITO). The dielectric layer includes a copper doped ZnS phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current.
EL phosphor particles are zinc sulfide-based materials, commonly including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors commonly contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce white light under an applied electric field.
Phosphor particles can be of many sizes, depending on the process and post-process treatment, e.g. milling. EL phosphor particles having an average particle diameter of 1-50xcexc, preferably 10-40xcexc, are typically used for screen printed and roll coated EL panels. Phosphor particles that are too large may interfere with formation of very thin phosphor layers, may result in grainy or nonuniform light output, and typically tend to settle too quickly from suspensions during manufacture. Phosphor particles that are too small may degrade more rapidly during use due to increased relative surface area, may agglomerate and not flow freely, and may be difficult to mix with binders in high loadings. The luminance of phosphor degrades with time and usage, more so if the phosphor is exposed to moisture.
It is known in the art to encapsulate phosphors with a moisture resistant coating to improve the performance of the phosphor. Encapsulated phosphor particles are coated with a substantially continuous coating of one or more metal oxides using a fluidized bed reactor. In particular, metal oxide coatings are produced by introducing appropriate precursors in one zone and hydrolysis with water vapor in another zone of the reactor. The fluidized bed maintains agitation and particle separation so coatings can grow on the surface of each particle and not join particles together. The metal oxide coating is substantially transparent and is typically between about 0.1-3.0xcexc thick, preferably between about 0.1-0.5xcexc thick. Coatings that are too thin may be permeable to moisture. Coatings that are too thick may be less transparent. For example, see U.S. Pat. No. 5,418,062 (Budd), U.S. Pat. No. 5,439,705 (Budd), U.S. Pat. No. 5,593,782 (Budd), U.S. Pat. No. 5,080,928 (Klinedinst), and U.S. Pat. No. 5,220,243 (Klinedinst).
An alumina coating formed as described in the Klinedinst patents tends to react with water to produce aluminum hydroxide (Al(OH)3), which deteriorates lamp materials. Eliminating the alumina coating is not desirable because of the benefits of the coating. U.S. Pat. No. 5,151,215 (Sigai) describes the problem of hydration/solubilization of the alumina coating on fluorescent phosphor particles and proposes heating the particles to a temperature of 700-850xc2x0 C. to cure the problem. EL phosphors cannot withstand such temperatures unaffected. Thus, the problem remains of overcoming a problem with a coating without giving up the benefits of the coating.
It is known in the art to coat the phosphor particles with polyureasilazane to improve adhesion and hydrolytic stability; e.g. see PCT published application WO 99/35889 (Kosa et al.). Although the process is quite effective, there are problems with disposal of waste solvent and with drying the wet phosphor particles.
It is known that chlorosilane compounds react with water or alcohol to form reactive silanols. Reaction of silanols to bond with an oxide-like surface (containing hydroxyl groups) is normally carried out in inert organic solvent or in the reactive alcohol serving as solvent; see A Guide to Dow Corning Silane Coupling Agents, 1985, Dow Corning Corp.
In view of the foregoing, it is therefore an object of the invention to improve the moisture resistance of an EL lamp.
Another object of the invention is to provide an improved process for coating coated phosphor.
A further object of the invention is to provide a new group of materials for coating phosphor particles.
The foregoing objects are achieved in this invention in which it has been found that providing a second coating on the phosphor particles improves the life, brightness, and moisture resistance of the particles. Specifically, the particles are treated in a fluidized bed reactor with alkyl or arylchlorosilane compounds, which substantially coat the particles and greatly improves the resistance of the phosphor to high temperature, high humidity environments. Volatile chlorosilanes, vapor transported in an inert carrier gas as a water-reactive species, produce siloxane directly on the surface of phosphor particles in a fluidized bed. A new group of surface-active siloxanes for use on coated EL phosphors has been discovered.